The present invention is related to telecommunications systems, such as wireless local area networks. More particularly, the present invention relates to the protection of information bits, being transmitted within a telecommunications system.
In response to a growing need for low-cost, short-range, high capacity radio links, the European Telecommunications Standards Institute (ETSI) has established a standardization project for Broadband Radio Access Networks (BRAN). One of the broadband radio access networks being developed under ETSI BRAN is HIPERLAN Type 2 (HIPERLAN/2). HIPERLAN/2 is a short-range, high data-rate system that offers high speed access (i.e., up to 54 Mbit/sec) to a variety of networks including Universal Mobile Telecommunications System (UMTS) core networks, Asynchronous Transfer Mode (ATM) networks and Internet Protocol (IP) based networks.
An important feature of HIPERLAN/2 will be the centralized medium access control (MAC) protocol, which is intended to provide an efficient use of the available spectrum. In accordance with the MAC protocol, an access point (AP), also referred to as a base station, controls channel access by assigning downlink and uplink timeslots to the various mobile terminals (MTs) with which it is communicating, wherein a MT receives data from the access point during a downlink timeslot and transmits data to the access point during an uplink timeslot.
A characteristic of HIPERLAN/2 is that the data is transported by protocol data units (PDUs). There are different PDU types. For instance, for transporting control information, there are control PDUs, and for transporting actual data, there are data PDUs, wherein each PDU of a certain type has a fixed size.
The HIPERLAN/2 standard specifies three system layers, namely, the physical layer, the data link control (DLC) layer comprising logical link control (LLC) and MAC, and the convergence layer (CL). The CL is the interface between higher layers and the DLC layer. For example, there may be a CL for TCP/IP, which segments the IP packets into data PDUs. The DLC layer adds header information before the PDUs are passed to the physical layer. While the following discussion focuses on data PDUs, it will be understood that the same applies to control PDUs.
The physical layer of HIPERLAN/2 will be based on orthogonal frequency division multiplexing (OFDM) and convolutional encoding. The granularity of data units on the physical layer is therefore an OFDM symbol. Depending on the subcarrier modulation scheme, e.g. BPSK, QPSK, 8PSK, 16QAM or 64QAM, the number of OFDM symbols needed to carry one PDU will be different.
Another feature of HIPERLAN/2 is that several physical layer modes will be provided. For example, the system may provide physical layer modes based on the aforementioned modulation schemes and convolutional codes for rates of xc2xd, {fraction (9/16)} and xc2xe. An important requirement for the DLC design is that the physical layer modes shall be designed such that each PDU fits into an integer number of OFDM symbols. Otherwise, capacity is wasted by using e.g. bit padding.
As an example, where 48 subcarriers are used for data and each PDU comprises 54 bytes, a BPSK modulation scheme with a code rate xc2xd is used. In this case, there are 48 bits carried by each symbol. Using a code rate of xc2xd for the 432 input bits (i.e., 54 bytes*8 bits/byte=432 bits) results in 864 encoder output bits without tail bits. These 864 encoder output bits are carried by exactly 18 OFDM symbols (i.e., 864 bits/(48 bits/symbol)=18 symbols). Because there is an integer number of OFDM symbols, no padding bits are needed. This is true for all other modes in HIPERLAN/2, so long as the tail bits are discarded.
It should be highlighted that the code rates xc2xd, {fraction (9/16)} and xc2xe, which are being discussed for HIPERLAN/2 are only precise when the tail bits are disregarded. This issue will be discussed further below.
Another WLAN system currently being standardized is the IEEE802.11 system. The IEEE802.11 system is being designed with a 5 GHz mode, which will have similar physical layer parameters with respect to HIPERLAN/2. However, the IEEE802.11 system is specifically being designed for transmitting IP packets by radio, where the protocol principles are similar to Ethernet; hence, the MAC protocol will be very different from HIPERLAN/2. In an IEEE802.11 system, for instance, IP packets, or segments thereof, having variable lengths are transmitted. The code rates which are currently being considered for IEEE802.11 are xc2xd, ⅔ and xc2xe.
An example of a flexible MAC frame 100 of HIPERLAN/2 is depicted in FIG. 1. As shown, the MAC frame 100 includes a broadcast control channel (BCCH), which contains information that is transmitted over the whole area (e.g., cell) covered by one AP. The assignment of logical channels to different MTs is transmitted in the frame control channel (FCCH), sometimes referred to as the resource grant channel. Accordingly, each MT knows the exact, dedicated time period in the MAC frame 100 when it is expected to receive a downlink burst and/or send an uplink burst. A random access channels (RACH) is located at the end of the MAC frame 100. A MT may request capacity by transmitting the request in its assigned uplink burst channel or via the random access channel.
The described MAC frame 100 illustrated in FIG. 1 should be understood as one possible arrangement of fields. In fact, the fields may appear in a different order. Furthermore, some of the fields in the MAC frame 100 may not appear at all, while others may be added. Regardless, the present invention described below is still applicable.
In each MAC frame field, data is transmitted from the AP to one or more MTs, or vice versa. A block of data which is destined for, or sent by, one MT is called a xe2x80x9cburstxe2x80x9d. Each burst comprises one or more PDUs. On the DLC layer, the concatenation of several PDUs may also be called a PDU train, or xe2x80x98cellxe2x80x99 train when the transmission of ATM cells is involved. On the physical layer, a preamble may be added at the beginning of each burst for synchronization and channel estimation purposes. If the channel access scheme is dynamic TDMA, the length of a burst is variable.
A convolutional code (CC) can be used to encode blocks of data. When CCs are used, tail bits (e.g. zero bits) are appended to the stream of information bits. The tail bits ensure that the encoding process terminates in a pre-defined state, e.g. in the zero state, thus providing protection for the last bits in a block. For a CC with constraint length 7, 6 tail bits are needed for termination. This results in additional redundancy. However, the code rate of a CC is often given without taking into account the tail bits. For example, the code rates xc2xd, {fraction (9/16)} and xc2xe, which are being discussed for HIPERLAN/2, do not include the tail bits. Therefore, the actual code rate is slightly lower due to the redundancy being increased as caused by the use of additional tail bits.
In a TDMA system with fixed timeslots, e.g., GSM, the timeslots have a fixed duration, and while the number of information bits may vary, the number of modulating bits remains fixed. This is often provided by a variety of puncturing schemes, one per physical layer mode. The tail bits are included in the design of the puncturing scheme which is specific.
IEEE802.11 is an ad-hoc network without regular frame structure. IP packets or segments thereof, which have variable length or more precisely a length being typically determined by the IP layer above the IEEE802.11 protocol, are transmitted. The encoding is performed based on a selected coding scheme with a code rate for the whole packet. At the end of the packet, the tail bits are appended and encoded like the data. The encoded data, including the tail bits, is mapped to OFDM symbols. The last OFDM symbol may not be filled totally, hence bit padding is applied.
HIPERLAN/2, and like systems, are characterized by the following features: the PDUs comprise a fixed number of information bits prior to channel encoding, disregarding tail bits. Moreover, there are multiple physical layer modes such that the PDUs map to an integer number of OFDM symbols, or some other physical layer units. Finally, the number of puncturing schemes is reasonably limited, for example, to three simple puncturing schemes corresponding to the rates xc2xd, {fraction (9/16)} and xc2xe, and the encoding is conducted either on a PDU-wise or a burst-wise basis.
Given the aforementioned features, accommodating the additional tail bits in the encoded sample stream without wasting an additional OFDM symbol, or without using an inconvenient (for implementation) high number of complicated puncturing schemes for all possible combinations of physical layer modes and burst lengths, in the case of burst-wise encoding, is problematic. The problem is illustrated by way of example in FIG. 2 for the case of PDU-wise encoding. As shown, each PDU may be mapped onto 4 OFDM symbols for the physical layer mode I with code rate xc2xe, and onto 6 OFDM symbols for the physical layer mode II with code rate xc2xd, respectively, where the physical layer mode I and the physical layer mode II are examples, and may not necessarily correspond with any HIPERLAN/2 mode. Since the code rates for all available physical layer modes are restricted to relatively xe2x80x9csimplexe2x80x9d code rates, such as xc2xd, {fraction (9/16)}, xc2xe, the only possibility is to transmit the tail bits using an additional OFDM symbol as illustrated. In the case of relatively small PDU sizes, as in HIPERLAN/2, a large amount of redundancy for defining the final state of the encoder will result. Alternatively, in GSM, one specific puncturing scheme is used for each mode. This approach is not attractive for implementation reasons, particularly because the PDUs comprise a rather high number of bits.
The problem is essentially the same with respect to burst-wise encoding. This is further illustrated by way of the example in FIG. 3, where a code rate of xc2xe is assumed. In the example illustrated in FIG. 3, a burst is either 2 or 3 PDUs in length. Thus, the PDUs contained in one burst are mapped onto an integer number of OFDM symbols and the additional tail bits would require an additional OFDM symbol for transmission. Regarding the use of many specific puncturing schemes, the number of required puncturing schemes would be equal to the number of physical layer modes times the possible number of PDUs per burst, which may be in the order of 512 schemes (i.e., 8 modes*64 PDUs/Burst), or higher.
To overcome the problem of employing additional OFDM symbols to transport the tail bits, a solution with the following features is needed. Tail bits are transmitted within the usual structure of bursts, consisting of fixed-length PDUs, without using additional OFDM symbols, or other physical layer units. The use of many irregular or complicated puncturing patterns over the whole PDU or burst is avoided. The number of puncturing schemes for the different physical-layer modes as well as for PDU-wise or burst-wise encoding is only slightly increased, when compared to the case without the additional tail bits.
The present invention involves a forward error correction technique for use in wireless telecommunications systems, where a first convolutional coding scheme is applied to the bits in a first portion of a data block and a second convolutional coding scheme is applied to the bits in a second portion of the data block, and where the second portion of the data block may encompass a part of the data block or the entire data block, including the first portion. The first and the second coding scheme may be implemented using the same convolutional coding scheme with a first and a second puncturing scheme respectively. While the first coding scheme employs a first code rate, the second coding scheme employs a second code rate that is higher than the first code rate. The higher rate makes it possible to incorporate the one or more tail bits into the second portion of the data block. In so doing, generation of additional symbols, such as orthogonal frequency division multiplexing symbols, can be avoided.
In accordance with a first exemplary embodiment of the present invention, the various advantages associated with the present invention are achieved with a method for protecting a block of information bits. The method involves appending a number of tail bits to the block of information bits and encoding the block of information bits, along with the appended tail bits, at a given code rate. A first puncturing scheme is then applied to a first portion of the encoded block of information bits and a second puncturing scheme is applied to a remaining portion of said encoded block of information bits.
In accordance with a second exemplary embodiment of the present invention, the various advantages associated with the present invention are achieved with a method for protecting a block of information bits. The method involves appending a number of tail bits to the block of information bits and, thereafter, encoding the block of information bits, including the number of appended tail bits. A first puncturing scheme is then applied to at least one portion of said encoded block of information bits, including the appended tail bits, and a second puncturing scheme is applied to the entire block of information bits, including the portion of the block of information bits to which the first puncturing scheme was applied.
In accordance with a third exemplary embodiment of the present invention, the various advantages associated with the present invention are achieved with a method for protecting a block of information bits. This method involves appending a number of tail bits to the block of information bits and encoding the block of information bits, including the number of appended tail bits. Thereafter, a xe2x80x9cnominalxe2x80x9d puncturing scheme is applied to the entire, encoded block of information bits, which includes said number of appended tail bits, and an xe2x80x9cadditionalxe2x80x9d puncturing scheme is applied to at least one portion of said encoded block of information bits.